The present invention relates to a fine grain milling method and machine and, more particularly to an air blasting method and machine, wherein the fine grain milling method and machine include a cleaning gas jet nozzle and a powder jet nozzle accurately transferring patterns of a mask covering predetermined surfaces of workpieces to enable precise milling of the workpieces. As an example, the present invention deals with a process for processing an air bearing surface of a thin film magnetic head needed to be milled at a high accuracy.
In, for example, Japanese Patent Application Laid-Open No. 63-22272, loose grains are mixed into a gas to make a gas-solid two-phase flow, with a nozzle being provided to inject the gas solid two-phase flow to workpieces. A suction nozzle is provided for suctioning a mixture of chips and blasting materials. In order to form concavities of predetermined dimensions on the work materials using the fine grain milling process, as shown in FIG. 2A, a mask is used to cover portions other than the concavities on surfaces of the workpieces before the workpieces are milled. A pattern of the mask, then, is transferred to the workpieces. If a powder jet nozzle injects the powder to mill the workpieces, the injected powder is likely to accumulate at edges of the mask. The accumulated powder covers over portions other than the mask on the surfaces of the workpieces, thereby resulting in the surfaces are masked on a wider area than the original mask. This result is disadvantageous that the fine grain milling process cannot accurately transfer the mask pattern to the concavities formed on the workpieces. FIGS. 2A, 2B, and 2c illustrate such an adverse blasting process. If the mask is a band of equal width, for example, an apparent width tends to become wider during blasting. The boundary between a concave and convex cannot be formed so as to be vertically perpendicular to the concavity, but assumes an unfavorable gentle slope. The mask likely accumulates the grains, particularly in corners thereof (FIG. 2C). The corners are shaped round, but.
If the grains are injected with the workpieces stationarily positioned relative to the powder jet nozzle, a a uniform removal of the grains cannot be achieved. Inner work materials are blasted deep, and outer workpieces are made shallow. For the reason, the depth of removal cannot be made uniform only by moving the work materials in one direction relative to the nozzle. In addition, if the workpieces are arranged on a circumference of a circular workpiece holder, which is revolved around its center, as shown in FIG. 1, the depth of removal is shallow with the circumference being outer and deeper with the circumference being inner because of difference of the outer and inner circumference distances. This is due to equal volume of removal per unit of time.
With such problems in the prior fine grain milling process as pointed out above, it has the disadvantage that it is not possible to fabricate precision devices, such as a thin film magnetic head which meets required specifications. The impossibility will be explained below by reference to FIGS. 3A and 3B. In the usual magnetic disk drive, the magnetic head is constructed so as not to levitate higher than 1 .mu.m by an air flow above the revolving magnetic disk. The magnetic disk drive may have an increased recording density with the dispersion of magnetic flux given by the magnetic head being reduced on the magnetic disk. The magnetic flux of the magnetic head is generated from a gap of an electro-magnetic transducing element in the magnetic head. The magnetic flux density at a point is high with the point being close to the gap. The magnetic flux also is reduced with the point close to the gap. That is, the recording density can be increased as a distance that is, levitation, between the magnetic disk and the gap of the magnetic head is reduced and as the magnetic flux is narrowed. In order to minimize and stabilize the levitation of the magnetic head, is necessary to make a machine mill wherein a shape of an air bearing surface of the magnetic is as accurate as possible. In forming the air bearing surface of the magnetic head as shown in FIGS. 3A and 3B, it is easy to mill such a linear shape as in FIG. 3A, using a grinder or similar means in mass production. Even if an attempt is made to obtain a shape of varying width as in FIG. 3B with the grinder, however, the shape has some portions remained not milled. This is due to the fact that the revolving grinder cannot form the complicated shape but the linear one.
The prior art fine grain milling machine includes rubbing parts in a milling chamber in connection with movement of the workpieces and nozzle during blasting. If the grains flutter in the milling chamber, therefore, they are harmfully brought into the rubbing parts thereby damaging the rubbing parts with the result that the rubbing parts have a reduced and shortened service life.